The invention relates to optical materials made from silicon nitride, to a process for their preparation by pyrolysis of polymeric silazanes, and to their use.
The silicon nitride-containing ceramic optical material is obtained by pyrolysis of polysilazanes, which are previously processed to give the desired moldings. A surface oxidic coating can be produced by the pyrolysis process.
The pyrolysis of polysilazanes to give silicon nitride/silicon carbide-containing ceramic material has already been described in the literature (Ceramic Bulletin, Vol. 62 (1983), 904-915). Polysilazanes are generally prepared using chlorosilanes as starting material, which are reacted with ammonia, primary or secondary amines or with disilazane (U.S. Pat. Nos. 4,540,803, 4,543,344, 4,535,007 and 4,482,669).
A further method of preparing polysilazanes is to react aminosilanes with excess ammonia or excess primary amine. The aminosilanes are themselves prepared by reacting chlorosilanes with amines (FR-A 1 25 83 423). For example, tetrachlorosilane SiCl.sub.4 and methylamine give the tetrakis(methylamino)silane Si(NHCH.sub.3).sub.4 : EQU SiCl.sub.4 +8 CH.sub.3 NH.sub.2 =Si(NHCH.sub.3).sub.4 +4 CH.sub.3 NH.sub.3 Cl
The aminosilane is subsequently reacted with excess ammonia, with all the methylamino groups being replaced by NH groups. Viscous to highly viscous polysilazanes are produced, which can be pyrolyzed to give silicon nitride-containing material in a ceramic yield of from 72 to 79% by weight. The disadvantage of this process is the use of large amounts of alkylamine, half of which reprecipitates as alkylamine hydrochloride in the preparation of the aminosilane. The polymers prepared from the aminosilane are viscous and therefore difficult to process; fiber production is not possible.
A large number of optical materials are known from the prior art. The materials used hitherto (for example glass, polysiloxanes, polymethyl methacrylates (PMMAs), etc.) all have the disadvantage that their possible uses are limited by the upper temperature limits. Thus, polymethyl methacrylates can only be employed up to a maximum of 80.degree. to 90.degree. C. and glass up to a maximum of 800.degree. C.
In addition, in fiber optics, a coating completely surrounding the fibers must be applied. The application of coatings of this type, for example by immersing the fibers in a solution containing a polymer in dissolved form or by special spinning processes, is expensive and complicated.